1. Field of the Invention
The invention relates to a method for controlling a stream containing a component, which takes part in a catalytic reaction and which adsorbes on the catalyst.
The invention is in particular directed to control of injection of a reductant to an exhaust gas, where formed nitroaen oxides are selectively, catalytically reduced and where the reducing component is adsorbed on or desorbed from the surface of the catalyst. Both reducing component and nitrogen oxides must be avoided in the catalyst effluent.
The invention is especially useful in purifying an exhaust gas from combustion engines in vehicles, where nitrogen oxides are reduced by ammonia possibly injected as an aqueous solution of ammonia or urea.
2. Description of Related Art
Purification of an exhaust gas is already performed in many ways. One way is disclosed in U.S. Pat. No. 6,427,439, where purification of exhaust gas is carried out by catalytically reducing the nitrogen oxides, NOx. The addition of the reducing agent is controlled by an electronic engine controller, EEC, which determines the amount of reductant to be added as proportional to the NOx concentration, when this concentration is above a predetermined value. When the NOx concentration is lower and the amount of adsorbed ammonia is less than the ammonia capacity of the catalyst, the amount of added reductant is a predetermined amount. The EEC uses signals from measurements of NOx concentration, NH3 concentration, temperature, engine speed and engine load for calculation of the amount of adsorbed NH3 on the catalyst surface and of reductant addition at the time of the calculation. However, this means many different measurements and calculations are involved in a vehicle where load of the engine increases and decreases frequently and quickly.
In U.S. Pat. No. 5,628,186 a further method for addition of a reducing agent is described. The addition is controlled by detecting operation parameters of the engine and catalyst in the exhaust gas and then determining from NOx measurement the required amount of NH3 and then adjusting this from the catalyst performance. The temperature is calculated from the performance of the engine. This requires several computing steps.
The injection strategy for reductant addition of the process of U.S. Pat. No. 6,119,448 uses similar measurements, furthermore, a reference engine is included in the calculations.
Also patent number U.S. Pat. No. 5,950,422 discloses an exhaust gas purification method. The similar calculations of amount of reductant to be added is performed by dividing the catalyst volume into several sub-regions, where after calculations for each sub-region are performed.
Though a great number of methods to control addition of reductant into exhaust gas are known in the art, there is still need to improve NOx conversion by controlled addition of a reductant in order to prevent detrimental leakage of the reductant into the environment.